Some trench structures include a dielectric liner filled with a doped semiconductor or an electrical conductor. For certain uses, the dielectric liner needs to support significant electrical field strengths, for example, when trenches are used for trench field type metal-oxide-semiconductor field-effect transistors (MOSFETs) or trench gate type MOSFETs which both operate at significant operating voltages. One way to sustain higher breakdown voltages for such trench structures is to increase the thickness the dielectric liner or use a relatively high refractive index liner material.